There is a rapidly expanding need for omni-directional radio frequency antennas (i.e., having a radiation pattern with substantially constant gain over approximately 360.degree. of coverage in the azimuthal plane and at substantially all degrees of elevation). Such a capability obviates the need for physically or electrically scanning a directive antenna in order to communicate with systems located in various directions. Such a capability also obviates the need for aiming a non-scanning directive antenna in the general direction of several radio frequency systems in an attempt to acquire adequate communications with all of them.
The desirability for omni-directional antennas is particularly apparent in mobile applications in which it is intended that the mobile radio system communicate with one or more different remote (mobile or fixed) systems as the mobile unit changes direction and location relative to the other systems. For example, it can be appreciated that a portable cellular telephone should have an omni-directional antenna to enhance the benefits of portability. It can be impractical, expensive or both to provide a mobile system with an antenna having electrical or physical scanning capabilities.
Broad bandwidth capability is another characteristic which may also be desirable in many of the applications in which an omni-directional pattern is desirable. Such a capability enables a communication system to operate over a number of different frequencies with a single antenna. Many existing antennas which are termed "broadband" are actually designed for operation at a selected center frequency; they may also have adequate performance over some range of frequencies on either side of the center frequency. To enhance performance over a very broad range of frequencies, however, means for manually or electrically tuning such an antenna should be provided. Making manual adjustments every time a frequency change is desired is inconvenient and not particularly reliable, as can be appreciated by anyone who has adjusted and tuned a TV antenna when changing stations. And, automatic electrical tuning requires the inclusion of complex and possibly expensive circuitry, making such an antenna impractical for many applications.
There are also many applications in which small size is a desirable feature, such as, for example, mobile applications in which the amount of space in which to mount an antenna is limited. For cosmetic, security and aerodynamic reasons, a low profile may be also desirable. It is preferable that the antenna feed (such as a coaxial cable connecting the antenna with a receiver or transmitter) not substantially affect the size or profile of the antenna. Therefore, it may be necessary to run the feed cable along one surface of the antenna itself. However, such a configuration can create coupling between the feed line and the antenna which can be detrimental to the performance of the antenna.
While numerous types of antennas have been proposed to address the foregoing desired characteristics, none have heretofore been able to adequately satisfy all of the characteristics in a single package.